It has been found, that in a rechargeable electrochemical power source provided with a pure metallic lithium negative electrode which cell contains an organic based liquid or polymer type of an electrolyte, that a reaction between this lithium anode and the electrolyte takes place that produces a thin passivating film over the surface of the anode that diminishes the electrochemical activity of the cell by causing a voltage delay on start-up after a storage period.
To counteract this passivating effect caused by the reaction between the lithium anode and the electrolyte, it has been suggested that an inert coating be produced in place on the surface of the lithium metal anode to insulate the anode from the electrolyte. Such a coating however must also be electrically conductive when in contact with the electrolyte. A nitride coating on the lithium has been found to be such a coating that effectively insulates the lithium metal of the anode from a direct contact with the electrolyte that produces the passivating coating. and yet the nitride coating is electrically conductive to such an extent as to not inhibit the electrochemical reaction in the cell when it has been put in use.
Various methods for producing such a desired lithium nitride protective coating on a lithium metal anode for use in an electrochemical cell have been mentioned in the prior art including nitriding the lithium by a solution deposition procedure, vapor deposition, and rapid quench techniques. These coating methods contemplate the application of the coating either by means of a simple lithium nitride deposition or by chemically reacting the surface of a lithium metal anode to produce the surface coating or electrically depositing a coating in-situ on the lithium metal to be used as the anode. In all of these known prior coating disclosures, for coating a lithium metal with a lithium nitride layer on an anode, the coating is described as being applied to the surface of an existing lithium metal that is then to be used for the anode.
It is known that lithium is a highly reactive metal and when exposed to the normal atmosphere quickly acquires a perceptible oxide coating. The existence of even a very thin layer of the oxidized lithium on the surface of the metal to be coated as taught in the prior art will interfere with the production of the nitride coating and with the ultimate serviceability of the hoped for protective nitride coating on the lithium metal anode being prepared for use in an electrochemical power cell.
The present invention provides an improvement on these known coating procedures for making lithium/lithium nitride coated anodes for use in electrochemical power sources that use organic based or polymer electrolytes.